Session: 18-05 Metallurgy, Coating and Repair I

Paper Number: 121186

121186 - Novel Alloying Strategy to Improve Brazing Properties on Nickel Based Superalloys for Aircrafts Turbine Application 

Superior repair technology is a crucial factor to extend lifetime and performance of aircraft turbine components. MRO processes allow to restore essential component properties whilst reducing resource consumption and carbon emissions. Especially diffusion brazing is a key technology for repairing cracks in high-pressure turbine components made of single-crystal Nickel-based superalloys. The Nickel-based brazing materials contain Boron as a primary melting point depressant that cause the formation of brittle phases (borides) as an undesired side effect.

The reduction of detrimental intermetallic brittle phases is crucial in achieving ductile brazed joints with high strength properties. This study seeks to design a novel alloying approach by focusing on Boron reduction. The approach is driven by identifying solutions to meet competitive targets. On the one hand side boron needs to be reduced for brittle phase reduction, but on the other hand side a reduction of Boron would result in increasing the melting temperature, reducing the viscosity and inhibiting the flowability characteristics. Flowability characteristics are essential for filling cracks by capillary action to form a defect-free joint. The new alloying strategy includes the utilization of multiple braze alloys that allows to separate functional braze alloy properties.

Based on thermodynamic modelling two new alloys were developed and verified experimentally. Thermodynamic modelling was carried out by the use of MatCalc® Version 5.61 (TU Vienna, Austria).  The optimization follows the 'alloys by design approach' according to Reed et al.. By using thermodynamic modelling software various approaches in substituting Boron by secondary melting point depressant elements, like Germanium and Titanium, were evaluated. Additionally, vital design criteria known for Nickel-based superalloys were considered to be included into the new design approach.

Experimentally the newly developed braze alloys are subjected to investigation in respect to wetting characteristics, capillary action and microstructural examination by light microscopy and scanning electron microscopy. The substrate for experimental set-up was chosen to be a Nickel-based superalloy that is commonly used for casting turbine guide nozzles in the high pressure turbine section of aircraft engines. The braze alloys developed in this study are produced by arc-melting in argon atmosphere. Pure elements for alloying are procured with at least 99.9 % purity. Diffusion brazing was carried out in a vacuum furnace at  mbar and under brazing temperatures conventionally known for repair brazing (1160 °C – 1230 °C). Temperature control was realized by using a PtRh-Pt S type thermocouple.

The new alloying strategy reduces the necessary boron concentration in the braze alloy by 25 % to 50 % in comparison to conventional nickel-based braze alloys, e.g. D-15. Experimental investigations indicate that the decreased boron concentration in the new braze alloy leads to a reduced boride precipitation while maintaining sufficient flowability characteristics. The new brazing technology is applicable to further Nickel-based superalloys and provides a basis to extend the repair capability of Nickel-based turbine components. The authors intend to perform tensile tests at elevated temperatures to investigate the microstructural improvement on mechanical properties.

Presenting Author: Dirk Wilhelm Reker MTU Maintenance Hannover

Presenting Author Biography: 2018, graduated as Master of Science in Industrial Engineering.

Since 2018, PhD student at Institute of Materials Engineering, TU Dortmund University in cooperation with MTU Maintenance Hannover (MTU Aero Engines).

Since 2022, Development engineer at MTU Maintenance Hannover (R&D) with focus on process development, high temperature brazing, joining of superalloys, single crystal welding, ceramic coating, high value repair technology for aerospace components

Authors:

Dirk Wilhelm Reker MTU Maintenance Hannover
Roman Sowa MTU Aero Engines Polska
Caspar Schwalbe MTU Aero Engines
Frank Seidel MTU Maintenance Hannover
Kai Moehwald Institute of Materials Science, Leibniz Universität Hannover
Martin Nicolaus Institute of Materials Science, Leibniz Universität Hannover
Steffen Wackenrohr Institute of Materials Science, Leibniz Universität Hannover
Wolfgang Tillmann Institute of Materials Engineering, TU Dortmund University